Re: [PATCH 1/7] xfs: take the ILOCK when accessing the inode core

Dave Chinner
2022-01-05T00:09:53Z

On Thu, Dec 16, 2021 at 03:56:09PM +1100, Dave Chinner wrote:
> On Wed, Dec 15, 2021 at 05:09:21PM -0800, Darrick J. Wong wrote:
> > From: Darrick J. Wong <djwong@kernel.org>
> >
> > I was poking around in the directory code while diagnosing online fsck
> > bugs, and noticed that xfs_readdir doesn't actually take the directory
> > ILOCK when it calls xfs_dir2_isblock. xfs_dir_open most probably loaded
> > the data fork mappings
>
> Yup, that is pretty much guaranteed. If the inode is extent or btree form as the
> extent count will be non-zero, hence we can only get to the
> xfs_dir2_isblock() check if the inode has moved from local to block
> form between the open and xfs_dir2_isblock() get in the getdents
> code.
>
> > and the VFS took i_rwsem (aka IOLOCK_SHARED) so
> > we're protected against writer threads, but we really need to follow the
> > locking model like we do in other places. The same applies to the
> > shortform getdents function.
>
> Locking rules should be the same as xfs_dir_lookup().....

Ok, I assumed the locking in xfs_dir_lookup() is optimal....

.... which it turns out it isn't. All calls to xfs_dir_lookup()
occur with the directory locked at the VFS level, so the internal
contents of the directory can never change during a lookup. Hence
holding the ILOCK across the entire lookup is both unnecessary and
excessive.

What xfs_dir_lookup() should be doing is what xfs_readdir() is
largely already doing - just locking the ILOCK around buffer read
operations when we are mapping directory offsets to physical disk
locations and reading them from disk. Changing this is a
significant set of changes, so it's not something that needs to be
done right now.

However, we still need to protect the xfs_dir2_isblock() lookup call
in xfs_readdir().

> > While we're at it, clean up the somewhat strange structure of this
> > function.
> >
> > Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> > ---
> > fs/xfs/xfs_dir2_readdir.c | 28 +++++++++++++++++-----------
> > 1 file changed, 17 insertions(+), 11 deletions(-)
> >
> >
> > diff --git a/fs/xfs/xfs_dir2_readdir.c b/fs/xfs/xfs_dir2_readdir.c
> > index 8310005af00f..25560151c273 100644
> > --- a/fs/xfs/xfs_dir2_readdir.c
> > +++ b/fs/xfs/xfs_dir2_readdir.c
> > @@ -507,8 +507,9 @@ xfs_readdir(
> > size_t bufsize)
> > {
> > struct xfs_da_args args = { NULL };
> > - int rval;
> > - int v;
> > + unsigned int lock_mode;
> > + int error;
> > + int isblock;
> >
> > trace_xfs_readdir(dp);
> >
> > @@ -522,14 +523,19 @@ xfs_readdir(
> > args.geo = dp->i_mount->m_dir_geo;
> > args.trans = tp;
> >
> > - if (dp->i_df.if_format == XFS_DINODE_FMT_LOCAL)
> > - rval = xfs_dir2_sf_getdents(&args, ctx);
> > - else if ((rval = xfs_dir2_isblock(&args, &v)))
> > - ;
> > - else if (v)
> > - rval = xfs_dir2_block_getdents(&args, ctx);
> > - else
> > - rval = xfs_dir2_leaf_getdents(&args, ctx, bufsize);
> > + lock_mode = xfs_ilock_data_map_shared(dp);
> > + if (dp->i_df.if_format == XFS_DINODE_FMT_LOCAL) {
> > + xfs_iunlock(dp, lock_mode);
> > + return xfs_dir2_sf_getdents(&args, ctx);
> > + }

Directory inode format cannot change here, so we don't need to
hold the ILOCK at all to do shortform checks.

> >
> > - return rval;
> > + error = xfs_dir2_isblock(&args, &isblock);
> > + xfs_iunlock(dp, lock_mode);
> > + if (error)
> > + return error;
> > +
> > + if (isblock)
> > + return xfs_dir2_block_getdents(&args, ctx);

Can the xfs_dir2_isblock() call be moved into
xfs_dir2_block_getdents() where it already takes the ILOCK to read
the block?

> > +
> > + return xfs_dir2_leaf_getdents(&args, ctx, bufsize);

Otherwise this patch is correct and this is where we should start
fixing the directory locking mess...

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com